Ethylene polymerization molybdenum catalysts

We begin with the structure of a noble metal catalyst, where the emphasis is placed on the preparation of rhodium on aluminum oxide and the nature of the metal support interaction. Next, we focus on a promoted surface in a review of potassium on noble metals. This section illustrates how single crystal techniques have been applied to investigate to what extent promoters perturb the surface of a catalyst. The third study deals with the sulfidic cobalt-molybdenum catalysts used in hydrotreating reactions. Here, we are concerned with the composition and structure of the catalytically active surface, and how it evolves as a result of the preparation. In the final study we discuss the structure of chromium oxide catalysts in the polymerization of ethylene, along with the polymer product that builds up on the surface of the catalyst. 

Edmund Field, Morris Feller, Promoted Molybdenum-Alumina Catalysts in Ethylene Polymerization, Ind. Eng. Chem., 49, 1883-1884 (1957) A. Zletz, Ethylene polymerization with conditioned alumina-molybdenum catalysts,... 

Standard of Indiana Catalyst. The first low pressure polyethylene catalyst invented (46), the Standard of Indiana catalyst system, saw relatively little commercial practice. Their 1951 patent discloses reduced molybdenum oxide or cobalt molybdate on alumina for ethylene polymerization, preferably in aromatic solvents. Later, work concerning the use of promoters was also disclosed. 

Other Early Developments. In addition to the breakthrough by Ziegler, two other discoveries of ethylene polymerization catalysts were made in the early 1950s. A patent by Standard Oil of Indiana, filed in 1951, disclosed reduced molybdenum oxide or cobalt molybdate on alumina (13). At the same time, Phillips discovered supported chromium oxide catalysts, prepared by impregnation of a silica-alumina support with Cr03 (14 16). Both the Phillips catalyst and titanium chloride based Ziegler catalysts are widely used in the production of high density polyethylene (HDPE). 

Sited with chromium in the same group 6B in the Periodic Table of Elements, molybdenum attracts many efforts in the study of ethylene polymerization. As early as in 1950s, Indiana Standard Oil Company discovered supported molybdenum oxide catalyst was active for ethylene polymerization (Field and Feller, 1957). However, the catalyst was then abandoned because of its poor catalytic performance. Our preliminary experiments on the MoO /Si02 catalyst also showed very low reactivity with comparison to the Cr0 y/Si02 catalyst for ethylene polymerization. In order to improve the activity of Mo-based catalyst for ethylene polymerization, the active valence states of molybdenum sites, and the mechanism of the catalytic reaction should be first elucidated. We performed a detailed theoretical study combined with experiments to investigate the active oxidation states of molybdenum and the effects of surface hydroxyl on the polymerization activity of supported Mo-based catalysts (Cao et al., 2010). 

Concurrent with the development of the Phillips process. Standard Oil of Indiana developed a similar ethylene polymerization process [14-17]. The basis of this process was the catalysis of ethylene to high density polyethylene using a supported molybdenum oxide catalyst under relatively modest conditions of temperature and pressure. The product has a range of densities similar to that available from the Phillips process. This system was not vigorously pursued and did not gain the acceptance of the Ziegler-Natta or Phillips processes. 

Molybdenum allyl complexes react with surface OH groups to produce catalysts active for olefin metathesis.34 35 Using silica as a support for the heterog-enization of Ti and Zr complexes for the polymerization of ethylene did not give clear results.36 In these cases, HY zeolite appeared to be a more suitable support. The comparable productivity of the zeolite-supported catalyst with... 

Scheme 5 suggests that every step of the ADMET polymerization cycle is in equilibrium and that, by shifting the relative concentrations of the condensate and polymer, depolymerization would result. In fact it has been shown that various unsaturated polymers can be depolymerized with excess ethylene, as well as substituted ethylenes. These depolymerizations can be done either with the tungsten or the molybdenum versions of Schrock s catalyst. 

Recent patent disclosures by the Standard Oil Co. of Indiana indicate that their process for the polymerization of ethylene is also a relatively low-pressure process, and the following process information is based on these disclosures. The polymerization process is a fixed-bed process employing a prereduced catalyst, ethylene pressures of 809-1,000 psi, and temperatures somewhat greater than 200°C. The metal oxides (such as nickel, cobalt, and molybdenum) can be supported on either charcoal or alumina, and materials such as lithium aluminum hydride, boron, alkali metals, and alkaline-earth hydrides may be used as promotors. Variations of this process are reported to produce polyethylene resins with densities from 0.94-0.97. 

The Standard Oil patent (16) describes a supported reduced molybdenum oxide or cobalt molybdate on almnina, with the ethylene preferably contacting the catalyst in an aromatic solvent to affect the polymerization. Operating temperatures of 100-270°C were disclosed and the molecular weight could be varied from very high to low such as those of greases. 

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